Birmingham Royal Orthopedic Hospital (BROH) CAM Offset: A Simplified Metric for Detection of CAM Morphology of the Hip

Abstract

Background CAM morphology of the hip is a disorder of abnormal morphology of the femoral head–neck junction associated with loss of the osseous sphericity of the femoral head. Several radiological measurements exist to detect CAM morphology of the hip.

Objectives The aim of the study was to evaluate the precision and efficacy of a new BROH CAM offset distance as a novel magnetic resonance imaging (MRI) parameter designed to detect CAM morphology of the hip.

Materials and Methods Oblique axial MR images of 100 patients who underwent hip MRI scans were analyzed. Two readers measured the alpha angle and BROH CAM offset to detect CAM morphology. Diagnostic cutoff values for cam offset measurements were determined, and their diagnostic accuracies were evaluated.

Results The mean “alpha angle” measured on an oblique projection was 34.31 degrees (standard deviation [SD] = 14.63), and when measured on an axial projection, it was 41.98 degrees (SD = 7.44). The BROH CAM offset distance was 3.49 mm (SD = 0.90 mm). There was an excellent correlation between the new BROH CAM offset and the alpha angle with a p-value of 0.006 (Pearson's linear correlation coefficient). Based on the results, a 57-degree alpha angle correlates to a BROH CAM offset of 4.11 mm, a 60-degree alpha angle correlates to a BROH CAM offset of 4.23 mm, and a 55-degree alpha angle correlates to a BROH CAM offset of 4.03 mm.

Conclusion Our new BROH CAM offset shows an excellent correlation with the alpha angle and is the easiest metric that can be used for the detection of CAM morphology of the hip.

Keywords

CAM morphology - alpha angle - femoroacetabular impingement - magnetic resonance imaging

Introduction

Femoroacetabular impingement (FAI) is a structural and functional condition in which abnormal contact occurs between the acetabulum and the femoral head–neck junction during hip movements. Three types of FAI are described in the literature: CAM, pincer, and mixed type. CAM morphology is characterized by an aspherical radial shape of the femoral head–neck junction, most frequently at its anterosuperior aspect, and most observed in younger men; pincer deformity represents over-coverage of the femoral head by acetabular overgrowth[1] and is more frequently observed in young to middle-aged women. Mixed deformities comprise a mixture of the two and are the most common type. CAM morphology may be observed as a physiological response to load bearing (particularly in young male athletes) and is considered a “primary” form. CAM morphology may also be secondary to healed fractures, slipped upper femoral epiphysis (SUFE), Perthe's disease, and preexisting hip arthritis.[2] In some patients, repetitive contact on these abnormal surfaces may lead to restricted range of motion, pain on exercise, and subsequent damage to the acetabular labrum and cartilage.

FAI can be assessed on anteroposterior radiographs as well as frog leg and lateral radiographs of the proximal femur. One can assess and evaluate the type of FAI, coverage and orientation on the acetabular side, as well as head sphericity, head–neck offset, and torsion on the femoral side.[3]

On conventional radiographs, lateral acetabular coverage is assessed by the lateral center edge angle (LCE angle), the acetabular index, and the extrusion index. The anterior femoral head coverage can be assessed by the anterior center edge angle on the Lequesne's false profile view.[3]

Multiple parameters have been proposed to measure CAM morphology including the alpha angle, femoral head–neck offset, and triangular index, which may be obtained on 2D radiographic and 3D cross-sectional imaging.[4] The alpha angle as described by Nötzli et al[5] is commonly used; an angle of ≥55 degrees is proposed to be significant enough to lead to a collision between the acetabular and femoral head surfaces.[6] [7] However, measurements and terminology may be inconsistent with considerable reader subjectivity.[8] In this study, we aim to compare the diagnostic utility and cutoff values of a new linear measurement, the BROH CAM offset with the traditional angled alpha angle.

Materials and Methods

Study Design and Population

The study was registered with the trust's clinical effectiveness unit. After receiving local ethical committee approval, the hospital's picture archival and communications system was searched to obtain a list of 100 patients who underwent MRI scans of the hip. This comprised 36 males (36%) and 64 females (64%). Proton density fat-saturated axial oblique MRI sequences (repetition time [TR]: 2,880; time to echo [TE]: 33; slice thickness: 3mm) were used to obtain measurements. The study excluded patients with postoperative evaluations, extensive trauma, prior history of developmental dysplasia of the hip or Perthe's disease, age above 60 years, and images degraded by motion and metallic artifacts. The images were assessed by a musculoskeletal radiology consultant and a musculoskeletal radiology fellow over 1 month, and another reader repeated the measurements after 2 weeks for intraobserver reliability. Patient demographic details, including age and sex, were recorded.

Measurements

The BROH CAM offset measurements were calculated using the following parameters.

The alpha angle was measured according to the description by Nötzli et al.[5] A best-fit circle was drawn to encompass the femoral head. Two lines were drawn from the center of this circle: (1) a line from the center of the femoral head to the femoral neck at its narrowest point and (2) a line from the center of the femoral head to the most medial margin of the anterior femoral head–neck junction, defined by a point at which there is a loss of spherical configuration of the femoral head. The angle between these two points was defined as the alpha angle ([Fig. 1]).[9]

Fig. 1 (A) Normal alpha and beta angle measurements. (B) High alpha angle in CAM type of femoroacetabular impingement (FAI).

The beta angle is defined as the angle between the line joining the acetabular rim and the center of the femoral head and the line from the center of the femoral head to the point where the spherical image of the femoral head is lost as reported by Wyss et al.[3] Several studies have been published regarding the same with a study by Mimora et al proposing a beta angle of 53.6 as an indicator of FAI on axial CT cuts in spine position with 60-degree rotation from the oblique axial slice.[10]

The new BROH CAM offset was measured on the same MRI sequence and slice as the alpha angle ([Figs. 2],[3],[4]).

Fig. 2 Line a is a straight line drawn from the anterior aspect of the femoral head to the anterior aspect of the greater trochanter. A perpendicular line b (BROH CAM offset) was drawn from this line to the anterior-most aspect of the femoral neck and the first line. (A) Normal BROH CAM offset measurements. (B) Low BROH CAM offset in CAM type of femoroacetabular impingement (FAI).Note: * denote CAM.

Fig. 3 (A) Proton density fat-suppressed axial oblique sequence showing measurements of the alpha angle and BROH CAM offset in normal (alpha angle 50 degrees and BROH CAM offset of 6.7mm) (B) and CAM type of femoroacetabular impingement (FAI; alpha angle of 60 degrees and BROH CAM offset of 2.3 mm).

Fig. 4 Proton density fat-suppressed axial oblique sequence showing an alpha angle of 58 degrees and BROH CAM offset of 6.78 mm.

A straight line was drawn from the anterior aspect of the femoral head to the anterior aspect of the greater trochanter. A perpendicular line was drawn from this line to the anterior-most aspect of the CAM bump and the first line. This measurement was defined as the BROH CAM offset distance.

The data analysis was conducted using STATA (Stata Statistical Software: Release 18, Stata Corp LLC, College Station, TX, United States). A linear regression was executed to examine the correlation between the new BROH CAM offset and the alpha angle.

Results

The study included 100 patients with 36 males and 64 females. The average age of the cohort was 34.3 years (range: 12–80 years) The mean “alpha angle” measured on an oblique projection was 34.31 degrees (standard deviation [SD] = 14.63), and when measured on an axial projection, it was 41.98 degrees (SD = 7.44). The mean offset distance was 3.49 mm (SD = 0.90 mm). There was an excellent correlation between the new BROH CAM offset and the alpha angle as measured on the axial projection with a p-value of 0.006 (Pearson's linear correlation coefficient). The regression between the two values was calculated as “Offset [mm] = alpha angle [degrees] * 0.04096 + 1.774” The best-fit line is presented in the figure with correlation and confidence interval represented ([Fig. 5]). Based on the results, a 57-degree alpha angle correlates to a BROH CAM offset of 4.11 mm, a 60-degree alpha angle correlates to a CAM offset of 4.23 mm, and a 55-degree alpha angle correlates to a BROH CAM offset of 4.03 mm. The correlation between the hip offset distance and the alpha angle as measured on the oblique plane was not statistically significant (p = 0.2475, Pearson). The alpha angle measured on the axial and oblique projections were statistically significantly correlated (p = 0.0032, Pearson).

Fig. 5 Graph showing the linear correlation between the BROH CAM hip offset distance and the CAM alpha angle.

The BROH CAM offset of 4.03 mm and an alpha angle of 55 degrees as measured on the oblique image (gold standard) were statistically correlated (chi-squared, p = 0.0072). A p-value less than 0.05 was considered statistically significant. There was good interobserver reliability with an intraclass correlation of 0.8.

Discussion

CAM morphology is characterized by anatomic loss of sphericity of the femoral head and most commonly occurs at the anterosuperior aspect of the femoral head–neck junction. CAM morphologies may be divided into primary (idiopathic) and secondary (developmental, traumatic and iatrogenic) morphologies; this study is primarily focused on primary lesions. It is a recognized but often underreported cause of hip pain in adolescents due to FAI. On examination, pain is reproduced by performing flexion, adduction, and internal rotation. However, it is recognized that CAM morphologies are often incidental and may remain indolent or asymptomatic in a large proportion of patients. The constant rubbing of the CAM bump against the acetabulum may lead to cartilage and labral degeneration characteristic following an “outside-in” pattern on MRI. In advanced cases, linear cartilage delamination “carpet tears” may ensue with premature osteoarthritis appearing within 2 to 5 years.[11] [12] It can sometimes be associated with pincer deformity, where there is acetabular over-coverage of the femoral head, leading to a “mixed type.”[13] [14] [15]

Early diagnosis and intervention are essential to prevent early onset of arthritis and subsequent decrease in quality of life. Close clinical correlation and exclusion of other causes (including unrelated labral tears, cartilage defects, muscular or tendinous pathologies, and synovial or joint-based pathologies) are essential to prevent overtreatment of an incidental CAM morphology. Symptomatic patients are often initially treated conservatively with analgesia, lifestyle modification, and physical therapy. Surgical treatment involves restoration of hip morphology through resection of the CAM bump[15] [16] and repair of any associated chondro-labral damage, where possible.

Traditional measurements to evaluate CAM morphologies include alpha angle, femoral head–neck offset, and femoral head-to-neck offset triangular ratio. Methods utilizing plain radiographs (anteroposterior and Dunn's view), CT, and MRI have been proposed to obtain these measurements.[17] This study compared how well the novel simplified BROH CAM offset distance correlated to the alpha angle measured on MRI as described by Nötzli et al,[5] which is an established and objective measure of CAM morphology. Our data set included 100 patients with a mean alpha angle of 41.98 degrees (SD = 7.44) and a mean BROH CAM offset of 3.49 mm.

The commonly used cutoff alpha angle value of 55 degrees corresponded to a BROH CAM offset of 4.03 mm. By using this cutoff, CAM morphology would be suspected where the BROH CAM offset is less than 4.03 mm.

Given that the BROH hip offset distance involves two steps, as opposed to the alpha angle involving three steps,[9] the readers found the BROH offset measurement to be easier and quicker to measure and this reduced the reporting time. A limitation of this study and previous studies is the variability of the location and orientation of CAM morphology. Lesions located more anteriorly, or posteriorly, may be suboptimally imaged on axial oblique MRI sequences. Furthermore, shallow but elongated CAM morphologies may be underestimated by the BROH technique. It is appreciated that the converse is also true, that is, short but deep CAM morphologies be underestimated by the traditional alpha angle measurement, as the reference point for the loss of circular orientation of the femoral head may be more lateral in location. A combination of both methods may more accurately characterize these lesions by encompassing both properties of the height and length of the CAM bump. One of the limitations is that the cohort for this study is small, and further studies would be useful to validate these findings.

Conclusion

The new BROH CAM offset distance demonstrates an excellent correlation with the traditional alpha angle measured on axial oblique MRI sequences of the hip. BROH CAM offset distance values of less than 4.03 mm demonstrated a statistically significant correlation to alpha measurements of greater than 55 degrees, a commonly used cutoff value for diagnosing CAM morphology. It may be useful as a supplementary tool in diagnosing CAM morphology.

Conflict of Interest

None declared.

Data Availability Statement

Data can be shared on request.

Ethical Approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Local ethical committee approval was obtained.

Consent to Publish

Yes.

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Publication History

Article published online:
27 March 2025